Electrolytic cells and cell membrane technologies have existed for many years. The function of electrolytic cells is to create aqueous caustic alkali products, such as caustic soda (NaOH).
The method by which the electrolytic cell creates aqueous caustic alkali products, specifically caustic soda, is as follows. Brine (or salt water) is used to create caustic soda, hydrogen gas, and chlorine gas. Referring to FIG. 1, electrolytic cell 5 has anode 15 and cathode 25 and cell membrane 30 between anode 15 and cathode 25. The use of cell membrane 30 creates anode chamber 10 and cathode chamber 20 within electrolytic cell 5.
Brine is fed into electrolytic cell 5 through line 50 into anode chamber 10. Water is fed into electrolytic cell 5 through line 35 into cathode chamber 20. When electric current flows through electrolytic cell 5, the chlorine ions in the brine water collect around anode 15 in anode chamber 10 as chlorine gas. Sodium ions from the brine collect around cathode 25 and reacts with the water to form caustic soda and hydrogen gas which collects in cathode chamber 20.
The chlorine gas and the depleted brine are removed from anode chamber 10 through line 42 and line 40, respectively.
The aqueous caustic soda.(or catholyte) and hydrogen gas is removed from cathode chamber 20 through line 45 and line 44, respectively.
However, the concentration of the aqueous caustic alkali products created by the electrolytic cells is normally not high enough to meet customer demands or to be used efficiently in other processes. Therefore, the aqueous caustic alkali product must be concentrated to a concentration level greater than the catholyte concentration in order to be acceptable to sell or use in other processes. For example, many customers require their aqueous caustic soda (or NaOH) to have a concentration of approximately 50% NaOH, but the concentration of NaOH coming from the electrolytic cell is approximately 32% NaOH.
The electricity used in the electrolytic cells to create the aqueous caustic alkali products releases heat which is absorbed by the materials within the cells, thereby raising its temperature. Thus, the temperature of the catholyte from the cathode chamber of the cell and the anoltye from the anode chamber of the cell have higher temperatures than the material entering the cells. Traditionally, the catholyte removed from the cathode chamber is split into two streams, one that is circulated back to the cathode chamber along with water added for dilution, and one that is to be concentrated and sold as product or used in another process within the facility. However, before the circulated catholyte can be returned to the cell, the heat added due to the electrolysis must be removed and the temperature of the catholyte reduced. This is most often done by the use of cooling water through a heat exchanger.
Further, in order to concentrate the aqueous caustic alkali stream, heat (typically from steam) is used to cause evaporation in order to remove the excess water. Boiling point rise is a physical property of every caustic alkali solution and increases with increased concentration and decreases with increased vacuum. Therefore, with the higher the concentration of caustic soda, the higher the temperature necessary in order to cause further evaporation of the excess water from the aqueous caustic solution.
The concentration of the aqueous caustic alkali has been done by several different methods including multiple effect evaporators, series of evaporators, or a single evaporator. Most plants use steam as a heating source in a multiple effect evaporator.
U.S. Pat. No. 4,090,932 by Kazihara discloses a method of recovering heat from the catholyte circulation line and using that as a heat source for the concentration process. However, the method disclosed will not work as described and cannot be easily modified without undue experimentation. There are several reasons why the design disclosed by Kazihara is unworkable or impracticable. Specifically, the circulated catholyte flow rate is excessive, the barometric condenser is incorrectly designed, and the catholyte heat exchanger is marginally designed.
First, the recirculated catholyte flow rate is excessive. In the disclosed process, the examples given require that the circulated catholyte flow is approximately 26 times greater than the catholyte flow to be concentrated. Current cell design requires that the circulated catholyte is less than 8 times the catholyte to be concentrated. Thus, the circulation required by the Kazihara design is more than 3 times that permitted by current cell design. The high circulated catholyte rate specified by the Kazihara design is impracticable since this flow is not acceptable for current cell technology.
Second, the barometric condenser is inadequately designed. Kazihara has either specified an incorrect Boiling Point Rise or has not allowed for a sufficient minimum temperature driving force for heat transfer for the barometric condenser. Boiling Point Rise is a physical property of any boiling liquid. Under atmospheric pressure water boils at 212° F. and the boiling temperature for 50% NaOH is 290° F., therefore, the boiling point rise of 50% NaOH is 78° F. (290° F.-212° F.). Furthermore, the water vapor evaporated under atmospheric pressure from the 50% NaOH can be condensed at 212° F. and this temperature is defined as the saturated vapor temperature (also referred to as dew point). Under high vacuum, where water boils at 95° F., the boiling temperature for 50% NaOH is 165° F. and the boiling point rise of 50% NaOH is 70° F. (165° F.-95° F.). The water vapor evaporated under this high vacuum from the 50% NaOH can be condensed at 95° F. (the saturated vapor temperature).
Kazihara specifies a boiling temperature of 74° C. (165° F.) for 50% NaOH in U.S. Pat. No. 4,090,932 FIG. 6 with the cooling water entering the barometric condenser at 30° C. (86° F.) and exiting at 34° C. (93° F.). In order to condense water vapor in a barometric condenser, the temperature difference between the exiting cooling water and the saturated vapor temperature must be at least 6° F. This indicates that Kazihara chose a Boiling Point Rise for 50% NaOH of 66° F. (165° F.-93° F.-6° F.=66° F. (37° C.)) rather than 70° F. which is supported by published data or that Kazihara selected a temperature difference between the exiting cooling water and the saturated vapor temperature of 2° F. that would use the correct Boiling Point Rise of 70° F. (165° F.-93° F.-2° F.=70° F. (37° C.)). Either the Boiling Point Rise has been incorrectly calculated or an inadequate temperature driving force has been specified. Regardless, the barometric condenser can not be designed as specified.
Third, the catholyte heat exchanger is marginally designed. Even if 50% NaOH is maintained at 165° F. (74° C.), the design disclosed by Kazihara is unworkable in practice. In FIG. 6 Kazihara specifies a catholyte temperature of 90° C. (194° F.) and a temperature to the evaporator (from the catholyte heat exchanger (6)) of 86° C. (187° F.). This means the temperature difference (ΔT) on that end of the exchanger is 194° F.-187° F.=7° F. The given circulation rates indicate that indeed Kazihara expects to maintain the 7° F. ΔT on both ends of the heat exchanger. Industry standard is to maintain ΔT of at least 10° F. on each end of the catholyte heat exchanger. A catholyte heat exchanger designed with a ΔT smaller than 10° F. tends to be excessively large and also difficult to operate.
In summary, this is an unworkable design which suffers from three major problems. Furthermore, any changes to fix one of these problems makes the other problems worse. All embodiments of the invention disclosed in Kazihara suffer from similar problems because in all cases they are trying to transfer the heat from the circulated catholyte to 50% NaOH.
U.S. Pat. No. 4,105,515 by Ogawa discloses a process for electrolysis of alkali halide in which the electrolytic cell is maintained at higher than atmospheric pressures and includes a multi stage double effect evaporator to concentrate the catholyte. First, current state of the art electrolytic cells do not operate above atmospheric pressure. Second, the disclosure only allows for the NaOH to be concentrated to 43% NaOH which is not economical to be sold as a product due to transportation and handling considerations. In most instances a higher concentration, of approximately 50% NaOH, is required. Third, the procedure disclosed will not function properly or give the desired results if the cell pressure is not higher than atmospheric pressure.
The present invention allows for decreasing the amount of steam to concentrate the catholyte generated from the electrolytic cell while recovering the heat generated by the electrolytic cell and maintaining a circulation rate of approximately 8 times the rate being concentrated. The first embodiment of this invention further allows for increased production when using prior equipment.